1. Field of the Invention
The present invention relates to a layer, particularly a surface layer, and more particularly a coating, for a suitable article, and relates to an article comprising the indicated layer. The invention further relates to a solution, particularly a solution comprising a coating composition for forming the indicated layer. The invention yet additionally relates to a process for preparing the solution, and to a process for preparing the indicated layer from the solution.
The present invention further relates to electrostatographic imaging and recording apparatus, and to assemblies in these apparatus for fixing toner to the substrates. The present invention relates particularly to the article and layer, as discussed, in the forms—respectively—of a fuser member, and a fusing surface layer for fuser members, in the toner fixing assemblies.
2. Description of Background and Other Information
Generally in electrostatographic reproduction, the original to be copied is rendered in the form of a latent electrostatic image on a photosensitive member. This latent image is made visible by the application of electrically charged toner.
The toner thusly forming the image is transferred to a substrate—also referred to in the art as a receiver—such as paper or transparent film, and fixed or fused to the substrate. Where heat softenable toners—for example, comprising thermoplastic polymeric binders—are employed, the usual method of fixing the toner to the substrate involves applying heat to the toner, once it is on the substrate surface, to soften it, and then allowing or causing the toner to cool. This application of heat in the fusing process is preferably at a temperature of about 90° C.–220° C.; pressure may be employed in conjunction with the heat.
A system or assembly for providing the requisite heat and pressure is generally provided as a fusing subsystem, and customarily includes a fuser member and a support member. The various members that comprise the fusing subsystem are considered to be fusing members; of these, the fuser member is the particular member that contacts the toner to be fused by the fusing subsystem. The heat energy employed in the fusing process generally is transmitted to toner on the substrate by the fuser member. Specifically, the fuser member is heated; to transfer heat energy to toner situated on a surface of the substrate, the fuser member contacts this toner, and correspondingly also can contact this surface of the substrate itself. The support member contacts an opposing surface of the substrate.
Accordingly, the substrate can be situated or positioned between the fuser and support members, so that these members can act together on the substrate to provide the requisite pressure in the fusing process. In cooperating, preferably the fuser and support members define a nip, or contact arc, through which the substrate passes. Also as a matter of preference, the fuser and support members are in the form of fuser and pressure rollers, respectively. Yet additionally as a matter of preference, one or both of the fuser and support members have a soft layer that increases the nip, to effect better transfer of heat to fuse the toner.
During the fusing process toner can be offset from the substrate to the fuser member. Toner thusly transferred to the fuser member in turn may be passed on to other members in the electrostatographic apparatus, or to subsequent substrates subjected to fusing.
Toner on the fusing member therefore can interfere with the operation of the electrostatographic apparatus and with the quality of the ultimate product of the electrostatographic process. This offset toner is accordingly regarded as contamination of the fuser member, and preventing or at least minimizing this contamination is a desirable objective.
Release agents, such as those comprising polydimethylsiloxanes, can be applied to fusing members during the fusing process, to combat toner offset. However, these agents may interact with the fusing member surface upon repeated use, and in time cause swelling, softening, and degradation of the fuser member.
Other factors also may disadvantageously affect the fusing member. Heat energy applied to this member can cause its degradation. Degradation can also be effected by continual contact with substrate toner, and by toner remaining on the fusing member surface. The fusing member surface can be subjected to abrasion by a variety of sources—such as the substrate, for instance, as well as elements of the fusing system, like the support member, release agent applicator, and contact heating members, if these are employed.
These unfavorable effects can result in an uneven fusing member surface and defective patterns in thermally fixed images. Where the substrate employed is paper, abrasion of the fusing member surface at the paper edge can form a worn area, or groove, that becomes problematic when the paper size is changed so that a larger paper overlaps the worn area. When the groove becomes deep enough to affect the fixing of the toner it causes objectionable image defects.
Where high image quality, and/or high image gloss, and/or controlling fusing member surface roughness are required, surface wear and abrasion are particular problems. For instance, if obtaining very high image quality is the objective, even a groove only worn into the surface enough to show a variation in surface gloss will nevertheless generate an objectionable defect in the image. Preventing or minimizing wear accordingly likewise is a desirable objective.
Heavily filled silicone rubber, used for fuser member surfaces, is known to produce high quality fused images. The polysiloxane elastomers have relatively low surface energies and also relatively low mechanical strengths, but are adequately flexible and elastic. Unfortunately, silicone rubbers wear easily when employed for this purpose; after a period of use, the action of the paper or other media passing through a high pressure nip wears a polysiloxane elastomer fuser surface. The silicone rubbers' low wear resistance as fuser member surfaces accordingly limits fuser member life. Further, although treatment with a polysiloxane release fluid during use of the fuser member enhances its ability to release toner, the fluid causes the silicone rubber to swell. This fluid absorption is a particular factor that shortens fuser member life; fluid treated portions tend to swell and wear and degrade faster. Fuser members with polysiloxane elastomer fusing surfaces accordingly have a limited life.
Fluorocarbon materials also have low surface energies, and, like silicone rubbers, are used as release surface materials for fuser members. Polyfluorocarbons employed for this purpose include nonelastomeric fluorocarbon materials, or fluoroplastics, and fluoroelastomer materials. However, there are disadvantages associated with the use of both.
In fact, the fluorocarbon resins like polytetrafluoroethylene (PTFE), and copolymers of tetrafluoroethylene (TFE) and perfluoroalkylvinylether (PFA), and fluorinated ethylene propylene copolymers, have excellent release characteristics due to very low surface energies. They also are characterized by high temperature resistance, excellent resistance to chemical interaction, and low wear (high abrasion resistance).
However, these materials are particularly susceptible to offset, due to high modulus and poor surface contact with rough substrates. Fluorocarbon resins also are less flexible and elastic than polysiloxane elastomers, and are unsuitable for producing high image quality images.
Yet additionally, fluorocarbon resins, having the indicated typically high modulus, cannot evenly contact rough papers, as noted. They therefore provide varying gloss within the same image.
This gloss variation may be referred to as mottled gloss. The poor contact, related (as indicated) to high modulus, also tends to produce images with objectionable offset.
Specifically, with a high modulus there will be objectionable mottled gloss as well as objectionable offset. Contact may be improved by the use of a thin fusing surface layer; however, a surface sleeve is limited to a certain minimum thickness when used in conjunction with an underlying soft cushion, because repeated compression results in sleeve wrinkling.
Fluoroelastomers, besides their low surface energy as indicated, have excellent wear resistance as fusing member surfaces. They provide better durability in this regard than the polysiloxane elastomers, and unlike the silicone rubbers, do not swell when in contact with polysiloxane release fluids.
However, fluoroelastomers have less resistance to chemical interaction than either silicones or fluoroplastics, and must be used in conjunction with reactive release fluids. As release fluids are subject to disruption or failure, fluoroelastomers are always at risk of irreversible contamination.
This is particularly a problem with polyester toners that may contain reactive sites on the toner surface. If the toner encounters the fluoroelastomer surface, the toner may chemically interact with the surface. If this interaction occurs the toner may not be easily removed, and will tend to attract more toner, leading to roller failure.
Inorganic fillers have been incorporated into fluoroelastomer surface layers to achieve the desired combination of properties like wear resistance, modulus, and thermal conductivity. Particularly, it is known that certain fillers may be used to reinforce the elastomer and further enhance the wear resistance of fluoroelastomers.
However, it is also known that the presence of inorganic filler particles, in the fluoroelastomer fusing surface layers of fuser members, provides high energy sites for removing toner from the substrate. In addition, inorganic fillers are typically extremely hard and abrasive to other elements of the toner fusing system that contact the fuser member.
It is further difficult to provide surface layers which are suitably free of defects, and which—in combination with high wear resistance—have a sufficiently high gloss, or are otherwise of the requisite degree of smoothness. Particularly it is difficult to provide surface layers with these desirable properties where the layers are obtained by application of the fluoroelastomer composition in solution, especially where the layers are built up to the desired thickness by applying successive coats.
Considering the foregoing, it would be desirable to provide a fusing member fluoroelastomer surface that retains the indicated advantages of fluoroelastomers, while also minimizing the tendency to acquire irreversible toner contamination. Especially with respect to fillers, it would be desirable that the tendency of the filler to cause toner offset be minimized, while the filler also enhances the wear resistance of the fluoroelastomer; further, it would be desirable to have fillers that do not wear contacting members.
In this regard, it is known to use PTFE as a filler for fusing surface layers. With PTFE and similar fluororesins being recognized as having low adhesion, good chemical resistance, and low coefficients of friction, there have been many attempts to combine these fluororesins with other materials used in fusing surface layers.
For instance, U.S. Pat. Nos. 3,669,707 and 3,795,033 disclose a fuser roller having a silicone elastomer surface that incorporates fluorinated resin filler, such as fibrillatable Teflon powder. U.S. Pat. No. 4,568,275 discloses a fuser roller with a surfacial layer prepared from an aqueous dispersion comprising fluorinated rubber and fluorinated resin. U.S. Pat. No. 5,376,996 discloses a fuser roller with a coating comprising a mixture of polyphenylene sulfide and polytetrafluoroethylene. U.S. Pat. No. 5,547,742 discloses a fuser roller having a surface layer comprising a fluorosilicone rubber and 5 to 50 weight percent of a fluororesin, such as polytetrafluoroethylene.
Further, U.S. Pat. No. 4,503,179 discloses an aqueous fluorine-containing rubber coating composition comprising a fluorine-containing rubber, a fluorine-containing resin, and an aminosilane. U.S. Pat. Nos. 4,555,543 and 5,194,335 disclose a film forming fluid coating or casting composition, comprising a fluoroplastic resin dispersion modified by a fluoroelastomer latex.
As with any polymer, the properties of PTFE are a function of, inter alia, molecular weight and method of preparation. And in fact, polytetrafluoroethylene can present different processing problems, depending upon the molecular weight of the PTFE being employed.
For instance, standard PTFE is an extremely high molecular weight polymer, on the order of 10,000,000, which cannot be melt processed due to the very high viscosity of the melt. Particularly for the purpose of the present invention, when polytetrafluoroethylene particles are subjected to shear, the desired result is for the particles to separate, and especially to disperse uniformly, or at least essentially uniformly. However, in this molecular weight range the PTFE is fibrillatable; when subjected to shear, it does not separate, but instead forms into fibers.
Specifically, granular powders which are composed of PTFE in this 10,000,000 molecular weight range rapidly undergo a process of fibrillation, which forms highly reinforcing fibers; these fibers make traditional processing methods inadequate. And the more the fibrillatable particles are sheared, the greater the fibrillation.
A reason for this fibrillation being especially undesirable—for the fusing surface layers of fuser members—is that many of the resulting fibers would be exposed, at the top of the layer. It has long been recognized that while PTFE is chemically inert, the surface may be adhered to by means of mechanical interlocking. Mechanical interlocking of toner melt with a fibrous surface would similarly be a concern.
Nonfibrillatable and autoadhesive polytetrafluoroethylene, in the molecular weight range of from about 25,000 to about 250,000, can be prepared by degradation of standard PTFE. One method which can be used to accomplish this degradation is mechanical shear. Another is chain scission by means of high energy bombardment.
The nonfibrillatable and autoadhesive form of PTFE can be dry compounded with fluoroelastomer. However, when thusly incorporated with the fluoroelastomer, only relatively small amounts of this PTFE can be used.
For instance, to avoid problems during milling and processing, the proportion of nonfibrillatable and autoadhesive polytetrafluoroethylene, particularly in the form of micropowders, that can be mechanically combined with fluoroelastomer is limited to about 25 percent by weight of the fluoroelastomer.
Further, if the dry compounded PTFE and fluoroelastomer composition is to be used for preparing a coatable solution, the proportion of PTFE must be still less. A proper solution can be obtained only where the nonfibrillatable and autoadhesive PTFE comprises about 5 percent or less by weight of the fluoroelastomer.
Specifically, with PTFE particles blended into a fluoroelastomer using dry blending methods, an amount of this PTFE higher than the indicated upper limit of about 5 weight percent results in a compounded composition that does not dissolve uniformly in appropriate solvents—particularly, a compounded composition that is unable to form a desirable coating. Attempted solutions, or dispersions, prepared from compositions with above about 5 weight percent of the nonfibrillatable and autoadhesive PTFE, will contain large agglomerates, and therefore be unsuitable for coating.
Ultralow molecular weight (ULMW) telomers of PTFE have been prepared synthetically and used as lubricants. As opposed to the foregoing nonfibrillatable and autoadhesive polytetrafluoroethylene, ULMW PTFE—having a molecular weight of from about 4,000 to about 25,000—can be used in amounts greater than the preceding weight percent limitations.
In this regard, U.S. Pat. No. 5,599,631 discloses a fuser member with an outermost layer that is a substantially homogeneous composite. This composite comprises a fluorocarbon elastomer as a continuous phase, and a fluorinated resin, having a molecular weight between about 4,000 and 25,000, as a discontinuous phase. The ratio of fluorocarbon elastomer to fluorinated resin is stated to range from 8:1 to 1:8. Compounding of fluorocarbon elastomer and fluorinated resin on a 2 roll mill, to obtain the desired composite, is disclosed; thusly obtained composites are dispersed in polar coating solvents, such as ketones and acetates, for coating on fuser members.
Accordingly, not only can ULMW PTFE indeed be compounded in high proportions with fluoroelastomer, but coatable solutions can be prepared from these high proportion ultralow molecular weight polytetrafluoroethylene compositions. However, the applicability of ULMW PTFE is limited. It is more expensive than the nonfibrillatable and autoadhesive PTFE, and is typically only available in the form of dispersions. In contrast, nonfibrillatable and autoadhesive PTFE is desirable, due to its lower cost, and availability in various particle sizes and dry powder forms.
Regarding the indicated difference between the properties of nonfibrillatable and autoadhesive PTFE and ULMW PTFE in fluoroelastomer—i.e., with respect to the much greater proportion of ULMW PTFE which not only can be dry compounded with fluoroelastomer, but also can be present in the dry compounded compositions used to prepare solutions—it is believed that the ultralow molecular weight polytetrafluoroethylenes are essentially pure crystalline compounds, which can fracture, but will not join together unless heated past their melting point. Thus, with ULMW PTFE particles in a medium, such as a fluoroelastomer or a solvent, shear will cause only the particles to disperse; even at high concentrations, with the shearing action bringing particles into contact with relative frequency, they will not join but instead will remain separate, so that the shearing action also moves them apart.
It is further believed that, in contrast, nonfibrillatable and autoadhesive PTFE has both a crystalline component and a substantial amorphous component that is cohesive—so that if PTFE particles are mechanically forced together with amorphous portions of different particles in contact, the particles will be joined by these amorphous components. Accordingly, if the PTFE concentration is high enough, the shearing action will bring amorphous components, of different PTFE particles, into merging contact with sufficient frequency so that particle masses will form.
U.S. Pat. No. 6,239,223 discloses a blended solid composition comprising a fibrillatable microparticulate polytetrafluoroethylene and a fluoroelastomer; also disclosed is a blended solid composition comprising a low molecular weight, nonfibrillatable polytetrafluoroethylene and a fluoroelastomer, wherein the nonfibrillatable polytetrafluoroethylene is present at greater than 35 percent by weight, based on total polymer solids of the composition. These blended compositions are isolated from an aqueous system by a low shear process. Use of a high molecular weight polytetrafluoroethylene, that can undergo fibrillation and thus reinforce the blend matrix, is indicated to be desirable.
While the indicated use of aqueous dispersions allows high levels of polytetrafluoroethylene in fluorocarbon elastomers to be obtained, aqueous dispersions limit both the curing methods as well as the range of additives. The surfactants required are also undesirable, particularly for fuser member applications. Additionally, fibrillation may be undesirable in a fuser member coating, as the wear surface would contain such strain induced fibers. It has long been recognized that while PTFE is chemically inert, adherence to PTFE particle surfaces may be effected by means of mechanical interlocking. Mechanical interlocking of toner melt with a fibrous surface would similarly be a concern.
Thus it is an object of this invention to provide a fusing surface layer that is uniform and defect free—or at least essentially uniform and at least essentially defect free—and that comprises a curable fluoroelastomer and a nonfibrillatable, autoadhesive polytetrafluoroethylene that has excellent resistance to paper abrasion and toner offset. It is further an object of this invention to provide a fusing surface layer that is of uniform—or at least essentially uniform, or at least substantially uniform—composition within, that is easily prepared, and that is resistant to shear during preparation. It is further an object of this invention to provide a surface layer that can be prepared from a compounded fluoroelastomer composition, and that is suitable for bisphenol type cure systems.
It is further an object of this invention to provide a method for producing a fusing surface layer having the desirable characteristics as indicated. It is further an object of this invention to provide a coating composition for producing a fusing surface layer having the desirable characteristics as indicated, with the coating composition also characterized by an exceptionally long solution life.